Grease and flour three 9-in. round baking pans; set aside.
In a large saucepan, combine beer, butter and chocolate. Cook and stir over medium-low heat until butter and chocolate are melted. Remove from heat; whisk in milk powder and cocoa. Transfer to a large bowl. Let stand 15 minutes.
Preheat oven to 350°. Add sour cream and eggs to chocolate mixture; beat until well blended. Combine sugar, flour, baking soda and salt; gradually beat into chocolate mixture until blended. Pour batter into prepared pans.
Bake 32-36 minutes or until a toothpick inserted in the center comes out clean. Cool 10 minutes before removing from pans to wire racks to cool completely.
In a large heavy saucepan, bring sugar and liqueur to a gentle boil over medium heat; cook until sugar is dissolved. Remove from heat. Add a small amount of hot mixture to egg yolks; return all to pan, stirring constantly. Cook 2 minutes longer or until mixture thickens, stirring constantly. Remove from heat; stir in vanilla. Cool to room temperature.
In a large bowl with a whisk attachment, cream butter until fluffy, about 5 minutes. Gradually beat in sugar mixture. Add milk powder; beat until fluffy, about 5 minutes. If necessary, refrigerate until frosting reaches spreading consistency.
Place bottom cake layer on a serving plate; spread with 1 cup frosting. Repeat layers. Top with remaining cake layer. Spread remaining frosting over top and sides of cake. Refrigerate at least 1 hour before serving.